Tolerance to the discriminative stimulus and reinforcing effects of ketamine

In order to examine whether tolerance develops to the discriminative stimulus and reinforcing effects of ketamine, rats were trained either to discriminate ketamine (10mg/kg) from saline or to self-administer ketamine (1.1mg/kg/injection), and then treated with chronic ketamine (32mg/kg), administered i.p. every 8 hours for 7 days. No shift in the dose-response curve for either paradigm was obtained following this chronic regimen. However, following a 2-week rest period in which animals had no exposure to ketamine, the dose-response curve was shifted two-fold to the left, indicating increased sensitivity to the drug. Reinstatement of training shifted the dose-response curve back to the right in both paradigms. These results suggest that tolerance to the discriminative stimulus and reinforcing effects of ketamine develops during training. Examination of the self-administration training data support this assumption, since inter-reinforcer time decreases, reflecting an increase in ketamine intake over training sessions.